Supply equipment for electric arcs



May 31, 1949- JL c. MLNE 2,471,641

SUPPLY EQUIPMENT FOR ELECTRIC ARCS Filed Jan. 4, 1949 Invenlor John C. Milne Attorney Patented May 31, 1949 UNH' SAT i3?? fr :o FFILCE SUPPLYr EQUIPMENT FOR ELECTRIC ARCS Company Application January 4, 1949, Serial No. 69,117 In Great Britain May 27, 1946 3 Claims.

l This invention relates to electrical installations for supplying and controlling arc lamps and has particular reference to such equipment for use 1-with projector arcs in cinemas and theatres.

' It is usual to provide two or more projectors which can be used to make it possible to change '.over'from 'one length of lm to another without any apparent break to the audience. This usually requires that as the end of one lm is approached,

Furthermore, owing to the properties of cinema arcs, it has been usual 'to provide a ballast resistance or choke coils in series with the arc A to stabilise the latter under working conditions.

The use of these, however, has the disadvantage of reducing the efficiency and one purpose of the present-invention is to render them unnecessary.

In such electrical supply equipment'fed from alternating current mains through a transformer and a Vapour rectier, it has been usual to employ separate transformers and rectiers for the two arcs in order to avoid any interaction or interference, since otherwise, if the rst arc is operating, on switching on the second arc, the first arc is liable to be momentarily dimmed and to produce an unfavourable effect on the illumination of the screen and on the audience. It is expensive to provide two sets of equipment and some saving has been eiTected by feeding both arcs through the same mercury vapour rectier which then would have six anodes in the case of a three phase supply, three anodes co-operating in the supply to each arc.

The present invention aims at simplifying still further the equipment by employing, in conjunction with a single mercury vapour rectier, a single transformer for both arcs and in such a way that the transformer itself provides the necessary ballast and that undesirable interference between the arcs does not occur, in spite of and each limb bears one phase of the primary winding with one phase of the two secondary windings on opposite sides of it. The desirable characteristics are given to the arc by arranging for the ballast to be produced by arranging the transformer coils with gaps between the primary and secondary windings adjusted duringfthe assembly of the transformer, so that there is considerable magnetic leakage. The primary winding will usually be delta-connected while the secondary windings will each be star-connected but with the phases 180 degrees out of phase and joined to two sets of anodes of the lmercury vapour rectiier so that the two sets of electrodes operate to rectify during the halves of the alternating current of opposite sign.

Both arcs are connected, on one side, to the cathode of the rectifier and each arc is connected, on the other side, to the neutral point-of the associated secondary winding of the transformer.

The invention will now be further explained with reference to the annexed drawing which is ideas on this subject which have been held up to acircuit diagram of an example of Vthe-system connected in accordance with the invention.

The three phase source of supply is Vshown as a star-connected three-phase alternator with the three phases shown at A, B and C and the neutral point at N. It supplies the mesh-connected primary winding P of the transformer employed. The number of effective turns of the winding P is adjusted by means of two 3-pole rotary switches with four positions providing sixteen tappings, the adjustment being course at one end of each phase and iine at the other, the tapping being shown at p. The secondary windings Sl, S2 are star-connected with the phases of each placed on the three limbs of the transformer core on oppcsite sides of the respective phases of the primary winding P and with gaps between the primary and secondary windings set by spacing washers during the assembling of the transformer. lt will be noted that the two secondary windings are connected for the two windings to produce voltages in phase opposition. Each secondary winding is connected to three of the anodes aal, abi, acl and aa2, db2 and ac2 of a six-phase vapour rectier R of the mercury pool type through three poles of a starting contactor SCE or SC2 and three poles of a running contactor RCI or RC2. contactors are excited from the phase C of the alternating source of supply through a stop button SS! or SSZ, a starting button SBI or SBZ and a running button RBI or RBZ.

If one of the starting buttons, for example,

The windings of the four SBI is closed, the starting contactor SCI is operated and closes a locking circuit at the contact Z1 and the circuit of projection arc AI is completed from the secondary winding SI, main contacts of contactor SCI, resistances TaI, rbi and rei to the anodes aaI abi and acl of the rectifier R to the cathode C, exciter relay ER, ammeter aI arc AI, cathode choke CI to the neutral point of the transformer secondary winding SI.

To start the rectiiier R so that current can flow in the above circuit, the carbons of the arc AI are brought together and a secondary starting winding SW oi the transformer supplies current through the rectier starting coil SC, through the exciter relay ER, ammeter aI, cathode choke CI, contacts eri of the exciter relay contacts eci of the contacter SCI and back to the starting secondary winding SW. The clipper D operates and the rectifier R strikes in the ordinary manner and the current flows through the rectifier R and arc AI by way of the ircuit described above. The relay ER is then energised and opens the contacts eri and thus disconnecting the 4starting circuit. Then on pressing the running button RBI, the

contactor RCI is operated and is locked at the contacts mi and short-circuits the resistances mi, Tbl and Tcl and then the rst arc AI is running normally. The contractors RCI and SCI are interlocked at the contacts ZI, making it impossible to operate the contactor RCI until the contactor SCI is closed. If the button RBI is closed by mistake before the button SBI, there is no connection from the phase C of the supply to the winding of the cathode RCI.

When a change-over has to be made, the second arc A2 is started up in a Way similar to that described for arc AI except that as the second arc is started before the first is switched off, the starting gear for the rectiiier bulb )does not operate and is only required initially when the first arc is struck; the circuit parts Corning into play have been marked with similar characters to those already referred to for starting the arc AI except that the suix 2 has been added. The

arc AI can then be shut down by opening the stop button SSI which then release the contactors SCI and RCI.

Each of the arcs AI and A2 has a Voltmeter VI or V2 connected across it to assist in the regulation of the length of the arc. Two transformer secondary windings FI, F2 similar to the Winding SW but wound on the other two limbs of the `transformer core are connected to supply a small three phase motor for driving a fan for cooling the bulb of the rectiiier R.

I claim:

l. An alternating current electric equipment for supplying a pair of projection arc lamps, comprising a transformer having two secondary windings connected to operate in phase-opposition and a mercury vapour rectiiier having two sets of electrodes establishing two distinct conductive paths in said rectiiier, said two sets of electrodes being connected in circuit respectively with said secondary windings and they projection arc lamps.

2. A three-phase alternating current electric equipment for supplying a pair of projection arc lamps, comprising a three-limbed three-phase transformer, each limb bearing one phase of the primary winding and one phase of two secondary windings one on each side of the respective phase oi the primary winding and a mercury vapour rectifier having two sets of three-phase electrodes establishing two distinct conductive paths in said rectifier, said two sets of electrodes being connected in circuit respectively with said secondary windings and with the projection arc lamps.

3, A three-phase alternating current electric equipment according to claim 2, wherein the phases of the secondary windings of said transformer are spaced from the respective phases of the primary winding by predetermined distances in order to establish the ballast action of maintaining a steady operation of the arc lamps.

JOHN CHARLES MILNE.

No references cited. 

